Part of church hidden in map secretly (4)
I believe the answer is:
apse
'part of church' is the definition.
('apse' is part of 'church')
'hidden in map secretly' is the wordplay.
'hidden in' indicates a hidden word.
'APSE' is hidden in the letters of 'map secretly'.
(Other definitions for apse that I've seen before include "Church's part" , "Semicircular recess in church" , "part of cathedral" , "where praetor sat" , "may be seen in church!" .)